<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .container{
            width: 400px;
            height: 400px;
            background: #ccc;
            border: 1px solid blue;

            /* 方式1：弹性布局 */
            /* display: flex;
            justify-content: center;
            align-items: center; */

            position: relative;

            /* 方式4：网格布局 */
        }
        .container div{
            width: 100px;
            height: 100px;
            background: red;

            /* 文本的居中 */
            text-align: center;
            line-height: 100px;

            /* 方式2：定位 */
            /* position: absolute;
            left: 50%;
            top: 50%;
            transform: translate(-50%,-50%); */

            /* 方式3：定位 */
            /* position: absolute;
            top: 0;
            left: 0;
            bottom: 0;
            right: 0;
            margin: auto; */

        }
    </style>
</head>
<body>
    <div class="container">
        <div>哈哈</div>
    </div>
</body>
</html>